At present, there are demands for ever higher recording densities.
In recent years, optical pickups that optically reproduce information have been developed with a “near-field” construction where the distance from the recording medium to the optical lens is 200 nm or below.
Near-field constructions are used with the object of raising recording density. This is achieved by reducing the size of the beam spot by raising the numerical aperture (NA) of the optical lens system and using a short-wavelength laser (which is to say, a blue-violet laser). This enables the track pitch and the widths and lengths of recording marks to be reduced.
However, the spot diameter of a beam used during reproduction needs to be set at around double the width of the recording marks to ensure that the recording marks can be properly read. Putting this another way, it is desirable to form the recording marks on a recording medium with a size that is no greater than half the size of the minimum spot diameter of a beam used for reproduction.
The following processes are usually performed during the manufacturing process for recording media, such as during the manufacturing process for a production plate used when manufacturing recording media. These processes are illustrated in the simplified cross-section given in FIG. 9. A photosensitive material layer 102 is formed by performing spin coating on a substrate 101, such as a glass substrate, that composes the production plate. This photosensitive material layer 102 is then exposed, in accordance with data to be recorded for example, to a laser beam 103 that is focused by a condensing lens 104. After this, the photosensitive material layer 102 is developed, and, for example, the areas of the layer that underwent a photosensitive reaction with the incident laser beam are removed, to form a pattern in the photosensitive material layer 102. Etching is then performed on the substrate 101 with the photosensitive material layer 102 as a mask, so as to form fine indentations and/or projections corresponding to the recorded data.
One characteristic of the photosensitive material is that the photosensitive reaction occurs rapidly when the material is exposed to a given amount of incident light or more, as shown by the step-like γ (gamma) curve in FIG. 10. Here, a case where the material is exposed to a laser beam for which the distribution of laser power is shown by the curve 201 in FIG. 11A is compared with a case where the material is exposed to a more powerful laser for which the distribution of laser power is shown by the curve 202 in FIG. 11B. While the effective photosensitive reacting region 202a of the photosensitive material layer 102 is slightly larger than the photosensitive reacting region 201a for the laser beam shown by the curve 201, this increase is not proportionate to the difference in laser power.
The above situation means that during the manufacturing process for a production plate used when manufcturing recording media, for example, if the substrate 101 is rotated so as to make the laser beam scan the photosensitive material layer 102 tracing a spiral route and the power of the laser is varied as shown by an illumination pattern shown by the curve 203 in FIG. 12A, exposed parts 102A are produced in the photosensitive material layer 102 on the substrate 101 in accordance with the illumination pattern, as shown in FIG. 12B. Developing results in the exposed parts being removed , for example. If etching is performed on the substrate 101 with the photosensitive material layer 102 as an etching mask, fine indentations and/or projections are formed in a stable pattern, as shown by the overhead view of an indentation 103 that can be used as a recording mark in FIG. 12C, for example.
However, when the above method is used, there is the following problem when performing focusing control of the exposing laser beam on the photosensitive material layer.
One method that can be potentially used to perform focusing control for the exposing laser beam is to detect return light that has been reflected back off the photosensitive material layer. However, since the photosensitive material layer is transparent for the laser beam used during this process, it is extremely difficult to perform focusing control by detecting the return light.
For this reason, when exposing the photosensitive material layer to laser light, a separate laser beam, such as a red HeNe laser, with a different wavelength to the exposing laser beam is usually used as a focus position adjusting laser beam.
However, since in this case a focus position adjusting laser beam is used along with the laser beam used for exposing the photosensitive material layer, the relative positioning of the two lasers must be set with high precision.
Also, while both laser beams are focused by the same condensing lens, if the two laser beams enter the condensing lens as collimated beams, the focal positions for the two lasers will be different due to the different wavelengths of the lasers.
As a result, it is necessary to adjust these focal positions in advance by providing a certain degree of bias. Since, as mentioned above, the red laser used as the focus position adjusting laser beam has a longer wavelength than the laser beam used for exposing the photosensitive material layer, its focal depth is wider. This means that it is difficult to adjust the focal position of the exposing laser beam, which is to say, the laser beam used to record data.
When two separate lasers are used as the focus position adjusting laser beam and the laser beam used to record data, there are additional problems for the recording equipment, such as increased complexity, increased size, and complicated handling.
The pattern of fine indentations and/or projections formed by exposing the photosensitive material layer in a given pattern is determined for the most part by the spot diameter of the laser beam to which the substrate is exposed. This means that it is not possible to form a pattern of fine indentations and/or projections in excess of the optical limits. As a result, even if the spot diameter of the reproduction laser beam is minimized as small as possible, the width of the recording marks cannot be reduced to half the spot diameter of the reproduction laser beam or less.
An electron beam lithography apparatus has been developed as a pattern exposing apparatus for exposing the photosensitive material layer. Such apparatus can form fine patterns, and thereby contributes to increases in the recording density. However, the lithographic operation has to be performed in a high vacuum, giving rise to the problem that an electron beam lithography apparatus is both large and expensive.
According to the present invention, when the process for manufacturing a recording medium or a production plate used when manufacturing recording media includes a process for exposing a material layer formed on a substrate, which composes the recording medium or the production plate used when manufacturing recording media, to a laser in accordance with a recording pattern, focusing control is performed without using a laser beam that is separate to the exposing laser beam as a focusing control laser beam. This solves the various problems that are listed above for the case where a focusing control laser beam is used.
Also, by making the width of recording marks equal to or smaller than the laser beam spot, an improvement is made in recording density.
Also, according to the present invention, an apparatus for manufacturing a recording medium or a production plate used when manufacturing recording media has a simplified construction, can be made small, is easy to handle, and is easy to maintain.